Skin hypersensitivity to the saliva of biting insects, specifically Culicoides species of midges. The itch may result in self trauma from biting, rubbing, and scratching enabling opportunistic pathogens like fungi and bacteria entry into the damaged skin surface, complicating the underlying condition with infections. The problem is seasonal associated with warm humid conditions, especially in the tropics.
Midges tend to swarm at dusk and dawn, or in humid overcast cloudy conditions at other times of day, so these are the risk periods for sensitive horses. Midges like water and moist conditions, so streams, creeks, and ponds as well as long grass in shaded areas under trees etc. are risk areas to be avoided if practicable. Bites can be anywhere on the body, but the itchy spots seem mainly to be concentrated on the face around eyes and lips, at the tail head, on the neck under the mane, and on the backline. Flanks and belly are less affected as a rule. Telltale signs are itching, rubbing, broken hair shafts and sores from rubbing.
Treatment may require alleviation of the itchiness in severe cases with corticosteroid creams or injections to reduce self-trauma. Topical treatment of concomitant skin infections may also be required using antiseptic or antifungal preparations.
Prevention revolves around avoidance of the times and places where exposure to midges is likely, stabling at dawn and dusk with the use of fans to discourage midge swarms, protection with rugs, face masks and insect repellants. Nutritional supplements with purported benefits for some horses include omega 3 oils, magnesium as Magnesium Sulphate, garlic to repel midges, and vitamin B1 supplements.
The condition cannot be cured completely as once sensitized, horses will always be prone to react strongly to midge bites.